How do I adjust for overexposure using physically based lighting?
Use the false colour look in combination with a CDL node (What is the the ASC-CDL node?) to bring your values within the range of the color transform. 

See also this related post: How to scale properly HDRI image, to be used in ASC-CDL HDR render?

and this one (somehow outadted, but should give you an idea): Using the False Color look in combination with the CDL Node to work on Wide Dynamic Range scenes